THIS invention relates to a method of producing an abrasive product containing diamond and cemented carbide.
Cemented carbide is a material which is used extensively in industry for a variety of applications, both as an abrading material and as a wear resistant material. Cemented carbides generally consist of suitable carbide particles such as tungsten carbide, tantalum carbide or titanium carbide, bonded together by means of a bonding metal such as cobalt, iron or nickel, or an alloy thereof. Typically, the metal content of cemented carbides is about 3 to 35% by weight. They are produced by sintering the carbide particles and the bonding metal at temperatures of the order of 1400° C.
At the other end of the spectrum, ultrahard abrasive and wear resistant products are found. Diamond and cubic boron nitride compacts are polycrystalline masses of diamond or cubic boron nitride particles, the bonding being created under conditions of elevated temperature and pressure at which the ultrahard component, i.e the diamond or cubic boron nitride, is crystallographically stable. Polycrystalline diamond (PCD) and polycrystalline cubic boron nitride (PCBN) can be produced with or without a second phase or bonding matrix. The second phase, when provided, may be, in the case of diamond, a catalyst/solvent such as cobalt, or may be a carbide forming element such as silicon. Similar sintering mechanisms are utilised in PCBN synthesis with various carbides, nitrides and borides being common second phases.
PCD and PCBN have a far higher wear resistance than cemented carbides, but tend to be somewhat brittle. This brittleness can lead to edge chipping of the working surface which can present a problem in applications where fine finishes are required. Furthermore, ultrahard products such as PCD and PCBN can generally not be directly brazed onto a metallic support. They are therefore often sintered in combination with a cemented carbide substrate. The bi-layered nature of such ultrahard products can be problematic in terms of thermo-mechanical stresses between the two materials: differential expansion and shrinkage on heating and cooling due to different thermal expansion coefficients and elastic moduli can lead to crack formation or unfavourable residual stresses if the substrate and the ultrahard products are too dissimilar. Another potential problem of such bi-layered materials is that of undercutting, i.e preferential wear of the less abrasion resistant carbide support. Further, machining of ultrahard products is difficult and costly, where carbide products can be relatively easily ground to the final geometry.
Efforts have been made to solve some of these problems.
U.S. Pat. No. 4,525,178 describes a composite material which includes a mixture of individual diamond crystals and pieces of pre-cemented carbide. The mixture is subjected to elevated temperature and pressure conditions in the diamond stable region, to create a composite polycrystalline diamond body. The mixture uses precemented carbide and not discrete carbide particles.
U.S. Pat. No. 5,045,092 describes a method of forming a cemented tungsten carbide article with embedded diamond particles. In this method, the embedded diamond particles are produced in situ.
European Patent No 0,256,829 describes a cemented carbide modified to the extent that it contains up to 20% by weight of cubic boron nitride particles. The cemented carbide is preferably produced under cubic boron nitride synthesis conditions so that damage to the cubic boron nitride is minimised.
European Patent No 0,583,916 describes a method of producing an abrasive product comprising providing a mixture of diamond and discrete carbide particles, the diamond particles being smaller than the carbide particles and present in the mixture in an amount of more than 50% by volume, and subjecting the mixture to elevated temperature and pressure conditions at which the diamond is crystallographically stable, in the presence of a binder metal capable of bonding the mixture into a hard conglomerate.